a) Field of the Invention
The present invention relates to a crystal growth technique for epitaxially growing a group II-VI compound semiconductor layer. The crystal growth technique of this invention is particularly suitable for growing crystals of, for example, mercury-cadmium telluride (HgCdTe) which is used for infrared detectors.
b) Description of the Related Art
Conventionally, HgCdTe has been grown on a bulk substrate such as CdTe and CdZnTe. It is necessary for an improvement on a resolution of an infrared detector to use a large area HgCdTe layer and form a number of pixels. However, it is difficult to form a large area bulk substrate such as CdTe and CdZnTe. In order to meet the requirements of a large area bulk substrate, it is necessary to grow a HgCdTe layer on a substrate having a lattice constant greatly different from HgCdTe, such as a GaAs substrate, an Si substrate, and a GaAs layer on an Si substrate.
In growing a HgCdTe layer on a substrate having a greatly different lattice constant, a CdTe buffer layer is used on which the HgCdTe layer is formed. However, a lattice mismatch between HgCdTe and the substrate such as GaAs is larger than a lattice mismatch between HgCdTe and a bulk substrate such as CdTe. Therefore, the HgCdTe layer grown on a different type substrate has a high dislocation density. Semiconductor devices made of crystals having a high dislocation density have generally poor electric and/or optical characteristics.
Japanese Patent Laid-open Publication No. Hei 3-196683 has proposed to forcibly curve, deflect or turn penetrating dislocation and suppress the propagation of dislocation by inserting a superlattice structure of HgTe/CdTe between a substrate and an HgCdTe layer. However, it is reported that because alternately growing two layers of HgTe and CdTe under the same condition is difficult, crystallinity becomes worse rather than directly growing a HgCdTe layer on a substrate, and the dislocation density becomes high (D. D. Edwall, J. Electronic Materials, 22, 847 (1933)).
Indium (In) bumps are generally used for the interconnection of an infrared detector of an array type formed on a HgCdTe layer to a signal processing circuit formed on an Si substrate. Since HgCdTe and Si have different thermal expansion coefficients, the infrared detector becomes likely to be peeled off from the Si substrate if the chip size is large.
As above, it is difficult for conventional techniques to grow a HgCdTe layer of a large area.